The exit row on a 737-300 is either Row 9 or 10, and has the best leg room on the plane.

Sometimes it will be in economy, sometimes business, depending on how busy the flight is for business class passengers. If you cannot get the exit row, then try to get the first row in economy class (the number will vary) in the "A" or "C" seats, as there will often be no "B" seat in that row (so you have more room)